This invention relates generally to cathode-ray tube (CRT) display systems and, more particularly, to dynamic focusing circuitry for CRT display systems.
As is known in the art, CRT display systems are used in a wide variety of applications. In CRT display systems without dynamic focusing circuitry, a projected electron beam is focused at a nominal position on the screen, typically the center of the screen. The beam, unfortunately, underqoes defocusing as it is moved to a position on the screen other than the nominal position. Defocusing is particularly pronounced in systems having substantially flat-faced display screens, due to the relatively large differences in the distance that the beam must travel from the electron gun to the correspondingly different points on the screen. In CRT display systems with dynamic focusing circuitry, however, circuitry is provided which attempts to maintain the beam in focus at each beam-directed position on the screen.
Various dynamic focusing circuitry has been described for raster-type display systems to maintain beam focusing as the beam is repetitively scanned at a predetermined rate across the screen. One such system, described in U.S. Pat. No. 3,412,281, uses the periodic beam position control signal to generate the focusing signal. The periodic beam position control signal is preamplified and biased to a predetermined DC level by a DC restoring stage. The DC level is chosen to provide proper beam focusing at the center of the screen, and the periodic variations from the DC level provide proper focus as the beam position is continuously and periodically varied from the center of the screen. To provide sufficient amplification to drive the focus electrodes, however, the preamplified and DC biased signal is fed to a relatively complex output stage which includes gated switching circuitry and associated transformer.
While such raster-type dynamic focusing circuitry may operate satisfactorily when used in some stroke-type display systems, in other systems the focusing circuitry may not provide satisfactory focusing due to, inter alia, the irregular manner in which the electron beam is moved about the screen in a typical stroke display system. More particularly, rather than being a regular, periodic signal having a single, dominant frequency component, as in a raster system, a stroke position control signal includes a spectrum of relatively high frequency components, corresponding to relatively rapid changes in beam position, in addition to both a spectrum of relatively low frequency components, corresponding to relatively slow changes in beam position, and a DC component, corresponding to the steady state focus signal when the beam is at a fixed beam position. Thus, the raster display dynamic focusing circuitry earlier described may not be suitable for stroke operation, since the inductance associated with its transformer might tend to suppress the high frequency components of the focusing signal associated with rapidly changing beam positions, thereby increasing focus response time and impairing tracking between beam focus and beam position when the beam position changes rapidly. Further, the system may not provide accurate focusing when the beam remains fixed at a single, off-center position on the screen, since the beam position control signal (a steady-state signal for fixed beam positions) is AC (capacitively) coupled in the focus circuit, and since the DC restoring stage changes the level of the steady-state signal to a predetermined level, namely, the level required to focus the beam at the center of the screen.
A dynamic focusing circuit for both raster and stroke display systems has been suggested in U.S. Pat. No. 4,258,298. The suggested circuit provides focusing for relatively rapid changes in beam position and maintains beam focusing even when the beam position is changed relatively slowly. This system uses a first circuit for generating a focus signal for slowly changing beam positions and a second circuit for generating a focus signal for rapidly changing beam positions. The outputs of the two circuits are combined into a composite focus signal in a frequency crossover network and such composite signal is added to a high-voltage DC bias signal and applied to a focus electrode to focus the beam. While this system may perform adequately in some applications, it is a rather complex arrangement, requiring sampling, amplification, rectification and feedback in the first, or low frequency, circuit, and transformer coupling in the second, or high frequency, circuit. Moreover, as described above with reference to U.S. Pat. No. 3,412,281, the use of a transformer in a circuit that is required to respond to rapid changes in an input signal may impair the tracking performance of the output to the rapid changes in the input signal.